fix(drivers): query real HVC block capacity instead of hardcoding 512 GiB

[AprilNEA]

The bug (permanent Docker corruption on VZ→HV switch)

arcbox_hvc_blk hardcodes the disk capacity:

/* Large default capacity — VMM validates sector range. */
set_capacity(disk, (sector_t)1 << 30);   /* 512 GiB */

"VMM validates sector range" only bounds I/O — the size the kernel and filesystem see is just wrong. docker.img is an 8 TiB sparse file whose Btrfs metadata is grown (btrfs resize max) to the visible device size on each boot:

• Under VZ, the guest sees the real 8 TiB → metadata total_bytes = 8 TiB.
• Switch to the HV backend → the guest sees only the hardcoded 512 GiB → Btrfs refuses to mount (open_ctree failed: … total_bytes should be at most 549755813888 but found 8796093022208) → dockerd never starts → Docker permanently broken (host persists backend=hv, so restart/reboot never self-heals). Data is intact; switching back to VZ recovers.

100% reproducible for any existing user who toggles HV.

Fix

Add ARCBOX_HVC_BLK_CAPACITY (0xC2000004) and query it per device in arcbox_probe_one, using the host's real capacity_sectors (already tracked host-side). Falls back to the previous fixed size when res.a0 <= 0, so old-host / new-guest keeps the prior behavior; new-host / old-guest is unaffected. The host half is arcbox PR #453.

Test plan

• Change mirrors the existing arcbox_hvc_flush HVC-call shape; CI builds arm64/x86 configs.
• End-to-end: with arcbox #453, boot the System VM under HV and confirm the guest disk reports 8 TiB and Btrfs mounts (needs a boot-assets release carrying this kernel).

[greptile-apps]

Greptile Summary

This PR updates the HVC block driver to report the real backing disk size. The main changes are:

• Adds the ARCBOX_HVC_BLK_CAPACITY HVC call ID.
• Queries each device capacity during arcbox_probe_one.
• Keeps the previous 512 GiB capacity as a fallback for older hosts.

Confidence Score: 5/5

Safe to merge with minimal risk.

The change is narrowly scoped and preserves old-host behavior when the capacity query returns a non-positive value.

No files require special attention.

T-Rex Logs

What T-Rex did

• The kernel-capacity-01-before proof shows that build-kernel.sh parses successfully.
• The kernel-capacity-source-proof log records the relevant driver definitions and call sites used for validation.
• The kernel-capacity-build-attempt log shows a build progression reaching driver injection but failing due to a missing flex tool.
• The kernel-capacity-02-after proof confirms that all capacity-query, fallback, and order checks pass with exit code 0.
• An accompanying Python source artifact is available for deeper review of the kernel-capacity checks.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
drivers/arcbox_hvc_blk.c	Adds a host capacity query for HVC block devices with the previous 512 GiB value retained as fallback.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
participant Guest as arcbox_hvc_blk guest driver
participant Host as ArcBox HV host
participant Block as Linux block layer

Guest->>Host: HVC ARCBOX_HVC_BLK_CAPACITY(idx)
alt Host supports capacity query
    Host-->>Guest: capacity_sectors in res.a0
else Old host or invalid result
    Host-->>Guest: "<= 0"
    Guest->>Guest: use ARCBOX_HVC_DEFAULT_SECTORS
end
Guest->>Block: set_capacity(disk, sectors)
Guest->>Block: add_disk(disk)

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sequenceDiagram
participant Guest as arcbox_hvc_blk guest driver
participant Host as ArcBox HV host
participant Block as Linux block layer

Guest->>Host: HVC ARCBOX_HVC_BLK_CAPACITY(idx)
alt Host supports capacity query
    Host-->>Guest: capacity_sectors in res.a0
else Old host or invalid result
    Host-->>Guest: "<= 0"
    Guest->>Guest: use ARCBOX_HVC_DEFAULT_SECTORS
end
Guest->>Block: set_capacity(disk, sectors)
Guest->>Block: add_disk(disk)

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_{Reviews (1): Last reviewed commit: "fix(drivers): query real HVC block capac..." | Re-trigger Greptile}

[linear-code]

ABX-467